A typical modern automobile includes an engine control system that provides closed loop fueling control. The control loop can include feedback paths that provide information from a number of exhaust gas sensors. These sensors generate respective signals that represent a predetermined combination of exhaust gas temperature and oxygen level, fuel/air ratio, or the like. Each sensor may be mounted in a respective housing, which is in turn mounted in a respective hole or mounting boss that allows the sensor to access the exhaust gas. Some implementations mount more than one sensor within a housing. This reduces the costs associated with making and assembling multiple housings and mounting bosses.
Referring now to FIG. 1, a cross section is shown of an exhaust gas temperature sensor 10 that is constructed in accordance with the prior art. Temperature sensor 10 employs a resistive thermal device (RTD) 12 that generates the exhaust temperature signal. RTD 12 is positioned on an alumina base 14. RTD 12 changes resistance based on the exhaust gas temperature. An engine control circuit senses the resistance and converts it back to an exhaust gas temperature. It is therefore important that the relationship between the resistance of RTD 12 and the exhaust gas temperature is known.
RTD 12 can be formed of platinum, palladium, and the like. Since the exhaust gas can reach temperatures greater than 1000 degrees Celsius, protection is needed for RTD 12. Compounds in the exhaust gas can alter the resistance of RTD 12, which causes the relationship between resistance and exhaust gas temperature to drift. A solution to this problem is to place an alumina cover 16 over RTD 12. Alumina cover 16 blocks the exhaust gas compounds from reaching RTD 12. Glass 18 bonds alumina cover 16 to RTD 12. At high enough temperatures, glass 18 becomes permeable. The exhaust gas compounds may then diffuse through glass 18 to RTD 12. In an environment that combines high temperature with lean exhaust gas, glass 18 in immediate contact with RTD 12 can cause the relationship between resistance and exhaust gas temperature to drift.
Referring now to FIG. 2, a cross section is shown of another embodiment of an exhaust gas temperature sensor 20 that is constructed in accordance with the prior art. RTD 12 is positioned on substrate 14. Glass 22 is inked on and fired. Glass 22 seals only the sides of cover plate 16 to the sides of substrate 14. This arrangement can sever the direct transport mechanism that exists between glass 18 and RTD 12 in the embodiment of FIG. 1. However, inking glass 22 to the outside edges of cover plate 16 can allow glass ink to seep underneath cover plate 16 and contact RTD 12. The glass will then cause the relationship between resistance and exhaust gas temperature to drift just as in the embodiment of FIG. 1.
The embodiments of FIGS. 1 and 2 both provide methods of using glass to bind a pre-fired alumina cover plate 16 to substrate 14. In both of the embodiments described thus far, the glass meant to form a barrier from the exhaust gas can become soft or permeable at high temperatures. As a result, even the glass allows the contaminants from the exhaust gas to reach RTD 12. Substitutes for the glass such as alumina ink, cannot generally be used to replace the glass ink because of a shrinkage mismatch; in order for alumina to be impermeable, it must first be sintered.